In the converter of a sulphuric-acid plant, sulfur dioxide (SO2) is converted to sulfur trioxide (SO3) by means of a catalyst. The gas containing sulfur dioxide is introduced into the converter together with oxygen, guided through a plurality of contact stages or trays of the converter arranged one after the other, and at least partly converted to sulfur trioxide by catalytic oxidation. The reaction of sulfur dioxide to sulfur trioxide is strongly exothermal, so that heat exchangers are provided between the individual contact stages, in order to dissipate the heat produced. To achieve a compact construction, one or more heat exchangers often are formed inside the converter, and the contact trays are arranged annularly around the central tube accommodating the heat exchangers. The construction and operation of a converter is described, for instance, in Ullmann's Encyclopedia of Industrial Chemistry, 5th edition, vol. A25, pages 649 to 653.
The individual contact stages of the converter are separated from each other by separating plates. In the contact stages, the catalyst usually contains vanadium pentoxide as an active component, is arranged on so-called trays. Converters for producing sulfur trioxide are usually very large for economic reasons. For instance, the vessel can have a diameter of 15 m. With a diameter of the central tube of 7 m, an annular width of 4 m is thus obtained for the trays carrying the catalyst. It is quite obvious that due the weight of the catalyst, the pressure loss of the process gas and the high temperatures existing in the converter, a high load is exerted on the trays, which leads to a plastic deformation. In general, it is assumed for instance in the case of austenitic steel that an elongation of about 30% leads to rupture. To avoid an excessive deflection and rupture of the trays, stiffening is required. For this purpose, a grid structure is usually incorporated in the converter, on which the trays configured as perforated plates are disposed, which are traversed by the process gas. Due to the high temperature caused by the exothermal reaction, which for instance in the first contact stage is about 625° C., it is necessary to use expensive stainless steels for the trays carrying the catalyst and the separating plates between the contact stages. Since the grid structure must also be made of stainless steel because of the high temperatures and must have a sufficient thickness to bear the catalyst weight, high weights and hence costs are obtained for these boiler inserts.